The subject matter disclosed herein relates to rotary-winged aircraft. More specifically, the subject disclosure relates to an airfoil section for at least partial use on a rotor blade of a helicopter.
Conventional rotary-wing aircraft have a forward airspeed limited by a number of factors. Among these is the tendency of the retreating blade to stall at high forward airspeeds. As the forward airspeed increases, the airflow velocity across the retreating blade slows such that the blade may approach a stall condition. In contrast, the airflow velocity across the advancing blade increases with increasing forward speed producing high lift, but increasingly higher drag that results in higher rotor power requirements. Forward movement of the aircraft thereby generates a dissymmetry of lift between the advancing and retreating sides of the rotor. This dissymmetry may create an unstable condition if lift is not equalized across the advancing and retreating sides of the rotor. An important approach in alleviating this dissymmetry is to use airfoils that are capable of producing high lift at high pitch angles and low relative velocities over the retreating side, while minimizing the increase in drag over the advancing side. However, designing such airfoils results in conflicting requirements as governed by the physics of the problem. That is, designing an airfoil that is capable of producing high lift at low speeds and low drag at high speeds typically results in the manifestation of some other undesirable characteristics, such as pitching moments that exceed the structural-dynamic tolerance of the rotor blades and control hardware.
Many airfoil sections have been developed, for example, as in US Patent Appl. Pub. 2007/0187549, that when applied to a main rotor of a helicopter, alleviate the unstable condition by addressing the lift and drag effects on the blade. These blades, however, exhibit high pitching moments which are detrimental to rotor blade dynamic characteristics.